This invention relates generally to devices for suppressing electromagnetic interference (EMI) in direct current machines, and more particularly, to the integration of a highly reliable, inexpensive suppression device into the structure of a brush holder. Such a brush holder is used to hold a brush of the type which makes a sliding electrical contact with a moving part.
Electromagnetic interference is commonly generated by dynamic electrical connections, such as the changing connections produced by the relative movement between the brushes and commutator of an electric motor or generator. The rapid changes in the electric and associated magnetic fields caused by the electrical switching EMI can interfere with the operation of other electrical systems, particularly digital systems operating at low voltage and low current levels.
Commonly, EMI problems are solved by adding noise suppression circuitry to each system which may be affected by the noise, rather than by reducing the noise at the source of the EMI. Such solutions are costly and result in circuit complexity since each system requires individual protection.
A more cost efficient and reliable means of solving the problem is to suppress EMI emissions at their source. Prior arrangements for suppressing EMI at the source have been made. For example, U.S. Pat. No. 2,453,114, issued May 16, 1945 to Brandt, teaches the integration of a capacitor into the structure of a brush holder. The integral capacitor/brush holder is comprised of a hollow casing made of insulative material and an outer electrically conductive layer acting as one capacitor plate. The capacitor shunts the high frequency EMI to ground through a low impedance path. While the Brandt patent teaches a means to suppress EMI at the source, it is neither a simple nor low cost solution. First, a good electrical connection to ground near the EMI source is necessary for effective capacitive filtering. Therefore, when a good ground connection is not readily available, or it is undesirable for some reason to inject EMI into ground, capacitive filtering is not necessarily the most effective EMI suppression technique. Second, the materials and manufacturing processes involved in constructing a capacitor of the type shown in Brandt are expensive, complex and time-consuming. For example, the physical dimensions of a capacitor are usually proportional to its capacitive value. Therefore, separate tooling and material processing may be required to construct Brandt casings of various capacitive values. As a result of the limited flexibility in constructing Brandt type casings, the flexibility to design the EMI filter to operate over specific frequency ranges is also limited.
Accordingly, there is a need for a highly reliable, inexpensive, improved suppression device integral to a small electric machine brush holder, to suppress the EMI commonly generated by such machines.